Add data-hide attribute to the bs-popover directive

This allows callers to hide the popover based on a scope variable.

Example usage:

    <button bs-popover="'/my/partial.html'" data-hide="foo">Click Me</button>

Any time $scope.foo becomes truthy, the popover will be hidden. Very handy if you
need to dismiss popovers based on the state of their enclosing scope.